swing-1-Tree与滚动条

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.tree.*;

public class test extends JApplet{
    JTree tree;
    JTextField jtf;
    public void init(){
        Container contentPane = getContentPane();
        contentPane.setLayout(new BorderLayout());
        DefaultMutableTreeNode top = new DefaultMutableTreeNode("Options");
        
        DefaultMutableTreeNode a = new DefaultMutableTreeNode("A");
        top.add(a);
        DefaultMutableTreeNode a1 = new DefaultMutableTreeNode("A1");
        a.add(a1);
        DefaultMutableTreeNode a2 = new DefaultMutableTreeNode("A2");
        a.add(a2);
        

        DefaultMutableTreeNode b = new DefaultMutableTreeNode("B");
        top.add(b);
        DefaultMutableTreeNode b1 = new DefaultMutableTreeNode("B1");
        b.add(b1);
        DefaultMutableTreeNode b2 = new DefaultMutableTreeNode("B2");
        b.add(b2);
        
        tree = new JTree(top);
        int v = ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED;
        int h = ScrollPaneConstants.HORIZONTAL_SCROLLBAR_AS_NEEDED;
        JScrollPane jsp = new JScrollPane(tree,v,h);
        contentPane.add(jsp, BorderLayout.CENTER);
        jtf = new JTextField("", 20);
        contentPane.add(jtf, BorderLayout.SOUTH);
        
        tree.addMouseListener(new MouseAdapter(){
            public void mouseClicked(MouseEvent me){
                doMouseclicked(me);
            }
        });
    }
    void doMouseclicked(MouseEvent me){
        TreePath tp = tree.getPathForLocation(me.getX(), me.getY());
        if(tp !=null)
            jtf.setText(tp.toString());
        else
            jtf.setText("");
    }

}